Recently, vehicle technology for the purpose of saving energy resources and preserving an environment has been developed. For example, there is a vehicle on which an idle stop system of cutting fuel supplied to an engine and removing torque generated by the engine when a predetermined condition (automatic stop condition) is established during operation is mounted. The automatic stop condition of the engine is established, for example, when a driver releases the foot from an accelerator or steps on the brake. Subsequently, the engine restarts at a point of time when a restart request from the driver is generated, or when the operation of the engine becomes necessary.
As a method of restarting the engine, there is adopted a method of using a starter of a pinion extrusion type to extrude a pinion of the starter to a ring gear side of the engine and engage the pinion with the ring gear, transmitting rotation of the starter to the engine, rotating the engine, and starting the engine.
For example, PTL 1 proposes a method of quickening recovery of rotation of the engine by starting to supply power to a motor of the starter and rotating the pinion when a condition such as stepping on the accelerator is established during inertial rotation after the torque generated by the engine is removed and a restart request is generated, engaging the pinion with the ring gear at a point of time when a rotation speed of the pinion is synchronized with a rotation speed of the ring gear, and starting cranking by the starter. PTL 1 discloses predicting a future rotation speed of the engine by operating kinetic energy of the engine and an amount of work that disturbs motion of the engine, and predicting future kinetic energy.